Butyl rubber which is a copolymer of isobutylene with a minor amount of isoprene is known for its excellent physical properties such as high weather-proofness, aging resistance, ozone resistance and heat resistance, favorable electrical properties and extremely low gas permeability. However, the rubber also has such defects as low elasticity, low rate of vulcanization and poor compatibility with other rubbers. As a means for eliminating such defects while retaining the foregoing advantageous properties, rather recently bromobutyl rubber was proposed.
As the vulcanizing agent of the bromobutyl rubber, sulfur, sulfur-donor, quinoid compound, resins such as phenolformaldehyde resin, metal oxides such as zinc oxide, and peroxides such as dicumyl peroxide are already known. Those known vulcanizing agents, however, invariably have one or other deficiencies as described below.
To wit, if sulfur alone is used as the vulcanizing agent, heavy mold staining takes place, and consequently the concurrent use of an acid acceptor such as lead oxide and zinc oxide is essential. However, it invites the elution of toxic acid acceptor during usage of the final rubber products, and inhibits the products' use in the fields of medicines and foods, for example, as bottles or stoppers. Also in the vulcanization with sulfur, the phenomenon of reversion is conspicuous, i.e., the rubber molecules are deteriorated when heated for a prolonged period at the vulcanizing temperature, to have impaired tensile strength and tension and increased elongation and stickiness. The occurrence of reversion can be confirmed by drawing the vulcanization curve using, for example, an oscillating disc rheometer, in which the torque reaches the maximum at a certain point of vulcanization, and thereafter decreases instead, as the vulcanization time is prolonged. As well known, normally the degree of reversion is great when sulfur is used as the vulcanizing agent, which not only renders the effective control of the operation extremely difficult, but causes qualitative deterioration of the vulcanized rubber obtained. Thus the prevention of reversion is keenly desirable. Furthermore, the vulcanization with sulfur causes notable color change of the vulcanized bromobutyl rubber to dark brown, and light colored rubber products cannot be formed therefrom.
As an attempt to inhibit the undesirable reversion in the vulcanization with sulfur, it is known to use a sulfur-donor, which releases active sulfur at the vulcanizing temperature, instead of sulfur. Although the reversion can be substantially avoided or markedly decreased by the substitution, there is produced another defect that generally the vulcanized rubber thereby obtained has reduced modulus. Furthermore, the problems of mold staining, necessity of using toxic acid acceptor to prevent the staining, and the objectionable color change in the vulcanized rubber remain unsolved.
In the vulcanization using quinoid, an activator such as red lead (Pb.sub.3 O.sub.4) must be concurrently used, which also is apt to be eluted from the final rubber products and due to its toxicity renders the products unfit for the usages in medicinal and food industries, like the aforementioned case. Furthermore, mold staining also takes place. In the vulcanization using the resin, again due to the toxicity of the resin which is used as the vulcanizing agent and may be eluted, the vulcanized rubber is deficient for use in medicinal and food industries. Elution of toxic matter is also not avoidable in the vulcanization using an oxide of heavy metal such as zinc, and therefore the method is undesirable from the view of environmental pollution. If a peroxide is used as the vulcanizing agent, metal oxide or hydroxide must be concurrently used as the acid acceptor in order to avoid the mold staining. Similarly to the case of sulfur vulcanization, the toxicity of such acid acceptor which may be eluted from the rubber products renders the method defective.